Specialize particle boundary types

[AhmedSalih3d]

Motivation

• Improve compiler specialization at the simulation boundary by making the particle StructArray shape concrete so the particle field tuple is visible to methods called from RunSimulation and neighbor loops.
• Reduce runtime dynamic field lookup and potential JET reports by threading a concrete particle container alias through hot entry points and kernels.

Description

• Add SimParticleStructArray alias in src/PreProcess.jl as const SimParticleStructArray{ParticleTuple, FieldArrays, IndexType} = StructArray{ParticleTuple, 1, FieldArrays, IndexType} and export it.
• Thread the SimParticleStructArray{ParticleTuple, FieldArrays, IndexType} type into signatures for RunSimulation, SimulationLoop, all NeighborLoopPerParticle! overloads, and ComputeInteractionsPerParticle! variants in src/SPHCellList.jl so the particle tuple and field arrays are concrete to the compiler.
• Specialize SetupVTKOutput in src/ProduceHDFVTK.jl to accept a typed StructArray (StructArray{ParticleTuple, 1, FieldArrays, IndexType}) and add the necessary StructArrays import.
• Re-export SimParticleStructArray from the package entrypoint in src/SPHExample.jl so downstream code can use the alias in signatures.

Testing

• Verified package loads with julia --project=. -e 'using SPHExample; println("loaded")', which succeeded.
• Ran the test suite with julia --project=. -e 'using Pkg; Pkg.test()', which errored: time stepping test raised MethodError for a Δt call (no matching 5-argument method); this failure appears unrelated to the particle-container specialization.

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Codex Task